Many applications of high temperature superconductors (HTSC) require the conductor to either withstand high mechanical stresses and/or strains during conductor fabrication or during use in application. Moreover, for many applications, a essentially non-magnetic substrate is desired at the temperature of application to eliminate or minimize AC losses from the substrate. For superconductor applications, a biaxially texture (i.e., cube texture) is also desirable. Biaxial texture, for the purposes of describing the present invention, is defined as follows:
The unit cells of all materials can be characterized by three co-ordinate axes: a, b, and c. The orientation of an individual grain in a polycrystalline specimen can be defined by the angles made by it's a, b, and c crystallographic axes with the reference specimen co-ordinate system. “Uniaxial texture” refers to alignment of any one of these axes in essentially all the grains comprising the polycrystalline specimen. The “degree of uniaxial texture” can be determined using electron backscatter diffraction or by X-ray diffraction. Typically, it is found that the grains have a normal or a Gaussian distribution of orientations with a characteristic bell curve. The full-width-half-maximum (FWHM) of this Gaussian distribution or peak, is the “degree of uniaxial texture” and defines the “sharpness of the texture”. Biaxial texture refers to a case wherein two of the three crystallographic axes of essentially all the grains are aligned within a certain degree or sharpness. For example, a biaxial texture characterized by a FWHM of 10°, implies that the independent distribution of orientations of two of the three crystallographic axes of essentially all the grains comprising the material can be described by a distribution wherein the FWHM is 10°. In cases wherein the material is characterized as cubic, the crystallographic axes a, b, and c are essentially perpendicular to one another. Biaxial texture of a certain degree essentially implies that all three crystallographic axes are aligned within a certain degree.
Currently, the preferred HTSC substrate material is a cube-textured Ni-5 at % W substrate having a yield strength of ˜150-175 MPa. This substrate is quite suitable for growing high quality epitaxial buffer layers thereon. The substrate is textured by successive cold-rolling to deformations greater than 95% via rolling followed by recrystallization annealing to form a sharp biaxial texture in the material of interest. Ni-5 at % W is however magnetic at 77K, resulting in deleteriously high AC losses (see for example, A. O. Ijaduola, J. R. Thompson, A. Goyal, C. L. H. Thieme and K. Marken, “Magnetism and ferromagnetic loss in Ni—W textured substrates for coated conductors,” Physica C 403 (2004) 163-171).
Moreover, cube-textured Ni—Cr substrates, including the non-magnetic Ni-13 at % Cr and Ni—Cr—W alloys, are non-magnetic and can be biaxially textured (see for example, J. R. Thompson, A. Goyal, D. K. Christen, D. M. Kroeger, Ni—Cr textured substrates with reduced ferromagnetism for coated conductor applications,” Physica C 370 (2002) 169-176). However, deposition of buffer layers is not straightforward as non-epitaxial Cr oxides form very easily during deposition of the seed layer and result in partial (111) seed layer orientations, resulting in misorientations in the superconducting layer. Also, the mechanical properties of NI—Cr and Ni—Cr—W alloys are weak with the yield strength only being about 150 MPa.
It is reported that Ni-9.0 at % W is an essentially non-magnetic alloy for all temperatures above 25K, having a saturation magnetism of less than 4.36 G-cm3/g (see for example Table 1 in A. O. Ijaduola, J. R. Thompson, A. Goyal, C. L. H. Thieme and K. Marken, “Magnetism and ferromagnetic loss in Ni—W textured substrates for coated conductors,” Physica C 403 (2004) 163-171). In comparison, as reported in this paper, the Curie temperature of Ni-5 at % W is 339K, that of Ni-6 at % W is 250K and that of Ni-9 at % W is 25K. Alloys containing Ni-9 at % W are very strong, having a yield strength of about 270 MPa, and are chemically quite suitable for growing high quality epitaxial buffer layers thereon. However, it is known that Ni substrates containing greater than 5 at. % W made by successive rolling at room temperature and subsequent recrystallization annealing undergo a deleterious texture transition that results in poor biaxial texture, making the substrates unsuitable for superconductor applications (see for example, V. Subramanya Sarma, J. Eickemeyer, C. Mickel, L. Schultz, B. Holzapfel, “On the cold rolling textures in some fcc Ni—W alloys,” Materials Science and Engineering A 380 (2004) 30-33).
The deformation texture of face-centered cubic (FCC) metals and alloys such as copper and nickel and their alloys can be of two types, either a “copper-type” texture (also called “pure metal-type” texture and denoted (123)[121]) or an “alloy-type”texture (also sometimes called “brass-deformation-type” texture and denoted (110)[112]). It is well known that the different deformation textures provide different recrystallization textures upon annealing. For example, the copper-type deformation texture is known to result (with appropriate annealing conditions) in a cube recrystallization texture, and the alloy-type deformation texture is known to result in a brass-type recrystallization texture. The cube texture is one of the preferred textures suggested in U.S. Pat. No. 5,741,377 referenced hereinabove.
As one or more solute or alloying elements A (such as Mo, W, Cr, V, Cu, Fe, Al . . . ) are added into copper or nickel (Ni1-xAx), and as the solute concentration x increases, it becomes increasingly difficult to achieve the copper-type rolling texture and then to obtain a cube or {100}<100>, recrystallization texture. In the range of a certain solute concentration, a gradual transition occurs with a mixture of textures, and above this concentration, one obtains primarily an alloy-type deformation texture which leads upon annealing to final brass recrystallization texture. The value of the transition solute concentration depends on the alloying element. Nickel and its low concentration alloys have high γ and give copper-type deformation texture, while as alloy concentration increases, γ steadily decreases, and above the transition solute concentration, a gradual texture transition with a mixed texture occurs and alloy-type deformation texture is increasingly obtained. The rate of decrease of γ with solute concentration x, and hence the value of the transition concentration, depends on the particular alloying element (see for example, “Recrystallization and Related annealing Phenomena” by F. J. Humphreys and M. Hatherly, 1995, pp. 328-329; “Structure of Metals” by Charles Barrett and T. B. Massalski, 1980, p. 558).
In the book titled “Recrystallization and Related annealing Phenomena” by F. J. Humphreys and M. Hatherly published in 1995 it is mentioned that for FCC metals and alloys which have a stacking fault energy, γ less than 25 mJm−2, an “alloy-type” deformation texture is obtained when rolling at room temperature. The “alloy-type” texture is also commonly referred to as the “brass-type” texture. For metals and alloys which have a stacking fault energy, γ greater than this value such as Cu with a γ less than 80 mJm−2 or Al with a γ of 170 mJm−2, a “pure metal-type” deformation texture is obtained. This pure-metal-type texture is also commonly referred to as the copper-type or Cu-type texture. A (111) pole figure of the pure metal and alloy-type texture is shown in FIG. 2.2a and FIG. 2.2b on page 44 of F. J. Humphreys and M. Hatherly. The stacking fault energy, γ of pure Ni is in between that of pure Cu and Al. By addition of alloying elements to pure metals such as pure Copper and Nickel, the stacking fault energy decreases. When the stacking fault energy decreases below a certain point, a texture transition in the deformation texture occurs from a pure metal-type to an alloy-type texture. The systematic and monotonic variation of stacking fault energy of Ni with addition of W, Mo, Cr and V, and many other solutes has been reported (see for example FIG. 17(b) of P. C. J. Gallagher, Met. Trans. Al (1970) 2429). The observation by Sarma et al. (V. Subramanya Sarma, J. Eickemeyer, C. Mickel, L. Schultz, B. Holzapfel, “On the cold rolling textures in some FCC Ni—W alloys,” Materials Science and Engineering A 380 (2004) 30-33) that above 5 at % W in binary NiW alloys, a mixed recrystallization texture is obtained upon heavy cold-rolling and annealing is consistent with the observations of Gallagher and Humphreys and Hatherly. The abstract of Sarma et al. states that the copper-type to brass-type texture transition in the rolling texture of cold rolled FCC Ni1-xWx alloys occurs at W contents >5 at. %. FIG. 5 of this paper confirms this statement and shows that above 5 at % W only a mixed texture or a partial “cube texture” is obtained. This implies that by successive cold rolling of NiW alloys above 5 at % W to deformations greater than 95% followed by recrystallization annealing will not result in a single orientation, cube texture. Only mixed textures are obtained using the conventional cold rolling followed by the recrystallization annealing procedure. Such mixed textures are of little value for any application where grain boundary misorientations are important. For example, depositing epitaxial buffer layers followed by epitaxial deposition of a superconductor layer will result in a mixed texture in the superconductor layer. The mixed texture necessarily implies numerous high-angle gain boundaries which suppress super-current flow. A superconductor with such high-angle grain boundaries will result in poor performance and of little use in applications. For obtaining good superconducting properties, at least a cube texture greater than 90% and preferably greater than 95% is required.
Schastlivetsev et al, Doklady Physics 49 p 167 (2004), teaches that binary alloys of Ni with Al, V, W, Cr and Mo all have a certain compositional range wherein only the Cu-type or pure metal type deformation texture is produced. It is suggested that one can also use the lattice parameter of the alloy to determine where the texture transition will occur. It is further suggested that, while texture development is a function of the specific rolling parameters and/or the starting grain size, alloys with lattice parameters greater than mid point of the mixed range, i.e. greater than 3.55 Angstroms, will have a mixed texture.
The problem heretofore unsolved is how to obtain a sharp cube texture in certain FCC alloys based on Cu and Ni which, upon cold rolling, exhibit a texture transition and result in some alloy-type or brass-type rolling texture components. Such alloys upon subsequent recrystallization annealing give a mixed annealing texture comprising of some brass-annealing components. It is important to note that alloys with high solute contents, such as Ni-9 at % W for example, are those which have desirable properties such as reduced magnetism and significantly increased strength. Magnetism of Ni alloys as a function of alloying additions has been extensively discussed. See Richard M. Bozorth, “Ferromagnetism” 8th edition, D. Van Nostrand Company, Princeton, N.J., 1951, pages 8, 269-270, 307-308, 320-321 and 325-326.